The oil production on an offshore environment, with increasing water depths, has become a real challenge in order to stabilize through mooring lines, the platforms or the floating units securely in operating position.
This is due not only by its own weight that these mooring lines reach due to the depth, also by all the drag influences they are exposed to because of sea currents, sometimes in different directions according to the depth, as well as by their own swing due by the waves on the sea surface and by the drag by winds acting on the floating unit.
The factors mentioned above as an example, together with the platform operational problems and also by the mooring manufacturing, may cause the breaking of the anchoring lines.
The disruption of one or more anchoring lines can lead the floating unit to a shift in position to beyond the safety margin and can cause very serious problems, particularly relating to the production pipeline connected to it, which may break leading to major environmental disasters as well as risk of death to the crew members.
The main guarantee of the integrity of the entire anchoring system, which maintains a given floating unit in a safe operation, is based on monitoring the tensile force wherein each of the anchoring lines are subjected to.
When they are being constructing, the platforms have as the integrated device, monitoring systems in order to monitor the anchoring lines that will eventually be linked to them. These integrated systems are subjected to the marine environment action which, over time, begin to provide mechanical and electrical failures and start to compromise the security as a whole. It is almost mandatory to use alternative device such that the monitoring of the stress tension in these mooring lines is performed.
It is very old the concern about the integrity of vessels anchoring lines, even with those simpler even with those that doesn't have a greater commitment than that of only to keep the vessel positioned.
The document GB 532 772 A of 1941 is a good example of this type of concern and means of prevention. This document discloses a link-shaped device that becomes part of an anchoring line, constructed with a central element sectioned. Electric contacts are fixed in each end of the sectioned central section of this element, being spaced from one another. These contacts are connected to a module which is laterally fixed to the link and houses batteries and a lamp. When the tension is excessive, the trend is that the chain link suffers an elongation deformation, and the two branches of the central element become in contact closing the electric circuit that lights the lamp and draws the attention of the involved personnel.
Over time the techniques has evolved to a more sophisticated devices, based on calculations made with data derived from correlations with phenomena suffered by a mooring line, for example, the existence of a relationship between the frequency of vibration of a certain line and the active tension force. In this technique, the device measure the natural frequency of a line vibration and the line tension may be calculated using the line specific weight and its free length between two points of support or attachment.
The document PI 0401668-8 A illustrates another tension monitoring mode or the disruption in submerged lines, and in general, among them, the anchoring lines. It comprises a device fixed to a region of a particular line with a sensor that is related to a parameter indicating a variation in the position of this device, and a communication unit for transmitting to a control base a variation that has occurred in said parameter. This method is positional and based on the fact that an anchoring line assume a certain inclination relating to the water line, and its angle is a function of the active tensile force acting on the line. The acting tension are obtained by calculations based on the measurement of the angles that are recorded by the sensor.
Another type of device for this purpose is shown in US 2013/0067881 A1 which shows an assembly of a device similar to the shackle, with a body in which one side is in the form of a fixed pin with arms extending in parallel in each end of this fixed pin. On the other ends of each one arms is interposed a pin. In at least one arm, a recess is formed inside which is housed a pressure sensor which acts as tensile meter.
Other examples are based on technical data of deformation or reduction in the width of a given link and on the changing of the steel electromagnetic properties when it is tensioned.
What is important to note is that some of the solutions presented above are permanently installed or are integrating part of the anchoring lines system, which are used as a component thereof. The technique lacks from a device for monitoring the tension in an anchoring line that is simple in its design, and which is adapted at one point of an anchoring line without being an integral part thereof, and thus, once collected the necessary data, the machine can be dismantled from this line and be mounted on another line, resulting in reduced costs.